Japanese laid-open patent publication No. 3-123269 (Reference 1) discloses a telephone exchange system which stores a voice of a ring-back tone or a busy tone in a telephone exchange and responds to a calling telephone with the voice.
Japanese laid-open patent publication No. 3-191645 (Reference 2) discloses a telephone capable of detecting a ring-back tone or a busy tone and outputting the detection result with a voice.
Japanese laid-open patent publication No. 9-162978 (Reference 3) discloses that images/voice messages to be sent from a subscriber to callers are preliminarily registered in a switchboard, and that the images/voice messages are sent to a caller before or during ringing when the subscriber receives calling.
Published Japanese translation No. 2003-505937 (Reference 4) discloses a technique to send commercial information in the form of voice/text/image instead of a ring-back tone.
However, none of References 1 to 4 pays attention to playing music via a portable terminal having a codec.
FIG. 1 shows a system having a conventional sound source supply apparatus 10. The system shown in FIG. 1 has the conventional sound source supply apparatus 10, a gateway switch (GS) 102, a mobile switching center (MSC) 103, a base station 104, and a cellular phone 105 as a portable terminal. Specifically, the cellular phone 105 is a GSM (Global System for Mobile Communication) terminal and has an audio codec 106 called an RPE-LTP (Regular Pulse Excited-Long Term Prediction) codec. The gateway switch 102 and the mobile switching center 103 form a switched network.
The conventional sound source supply apparatus 10 has a sound source file storage device 100 for storing a sound source file (later referred to as an original sound source file) 201 in a 128-kbit/sec WAV format (Windows® standard sound file format, also called WAVE) and a sound source playback device (dial melody device) 101.
The sound source file 201 is sent as a μ-Law PCM (Pulse Code Modulation) signal or an A-Law PCM signal of 64 kbit/sec by the sound source playback device 101. The gateway switch 102 and the mobile switching center 103 directly transmit the sound source file at 64 kbit/sec. Accordingly, no degradation of sound is caused to the sound source file in the gateway switch 102 and the mobile switching center 103.
The signal is converted into a wireless area signal with the same audio codec (RPE-LTP codec) 107 as the audio codec 106 of the cellular phone 105 by the base station 104. In a case where a user only listens to music, the audio codec 107 of the base station 104 converts the PCM signal into a GSM encode signal, and the audio codec 106 of the cellular phone 105 converts the GSM encode signal into the PCM signal. The cellular phone 105 converts the PCM signal into an analog signal and plays the music with a speaker for an owner of the cellular phone or the like. In a case where the user speaks, the cellular phone 105 performs a conversion opposite to the conversion for listening to music. In this case, the cellular phone 105 converts a voice from a microphone into a PCM signal, and the audio codec 106 of the cellular phone 105 converts the PCM signal into a GSM encode signal. The audio codec 107 of the base station 104 converts the GSM encode signal into the PCM signal.
Because a band between the base station 104 and the cellular phone 105 is narrow, an encode process specialized in human voice is performed. Accordingly, some frames in music are not regarded as a voice but as a noise. Thus, since noise frames are transmitted instead of the music, the quality of sound is degraded.
In other words, the codec 106 specialized in voice is used in the conventional sound source playback because of a narrow transmission band between the base station 104 and the cellular phone 105. Accordingly, there is a problem that the sound quality of music cannot be maintained.
This problem in the conventional sound source playback will be described in greater detail.
In response to a playback request of music, a prepared sound source file 201 is connected to the cellular phone network (including the gateway switch 102, the mobile switching center 103, and the base station 104) in an audio mode (64 Kbit/s, PCM) and played. In the cellular phone 105, particularly in a narrow-band cellular phone, degradation of sound is caused by codec characteristics of the wireless area. Accordingly, there is a problem that music cannot be provided with high quality.
The audio codec 106 used in the cellular phone 105 is designed to transmit only human voices even in a narrow band. Thus, the audio codec 106 employs an encode method specialized in human voice. On the other hand, music is considered to partially include white noise. Thus, since music is different from human voice, a waveform of music is suppressed so as to degrade the quality of sound.